Multilayer board panels or production pieces are formed by laminating a plurality of individual layers of board panels together. Each multilayer board (MLB) panel has an electrically non-conductive substrate material having conductive patterns formed thereon to provide electrical interconnect paths. Such paths lead to conductor terminations known as solder pads. In order to provide electrical connections between the different board layers, through-holes are drilled through a vertical stack of solder pads in the different board layers and are plated with an electrically conductive material such as solder. Ideally, all solder pads of a vertical stack should be vertically aligned so that it is possible to drill through the center of each pad in the stack.
While the individual circuit boards may be formed very precisely, distortions introduced during the lamination processing result in a misregistration of the different MLB panel layers such that the solder pads from the different layers may not be in perfect vertical alignment. The relative misregistration of one layer to another reduces the area that the pads overlap, making the effective drill target smaller. The minimum radius of the effective drill target must be greater than the drill placement tolerance or drill breakout will occur. Drill breakout occurs when a drill hole is not completely contained within solder pads on all layers of an MLB panel and is normally considered a rejectable board defect.
In order to determine the drill targets to compensate for the misregistration of the different layers, a drilling template has been used to locate the position of the holes. The drilling template is a metal plate that is drilled with a pattern of holes or fiducial shapes that correspond to the locations of fiducial pads on the MLB panel. Fiducial pads are pads that are located on the MLB panel for inspection purposes but do not form a part of the electrical connections and are not drilled. The template is positioned on an x-y table of an automatic x-ray inspection system in the same position as will later be taken by the MLB panel. The x-y table is then moved and the position of all of the template holes or fiducial shapes are recorded relative to some fixed reference point within a computer memory. After the positions of the holes or fiducial shapes are recorded, the template is removed and an MLB panel is placed on the x-y table of the x-ray inspection system. The x-ray inspection system then records the positions of the fiducial pads on the MLB panel, again relative to the fixed reference. By comparing the positions of the holes or fiducial shapes in the template with the positions of the fiducial pads on the MLB panel, the offset between them may be calculated. This offset can then be used by a drilling machine to determine the correct position for the drilled holes through the vertical stacks of solder pads in the MLB panel.
One significant disadvantage of this approach is that the absolute position of both the template and the MLB panel must be known. This is because the template holes or fiducial shapes are recorded separately from the fiducial pads of the MLB panel. Since absolute position must be known, the motion of the x-y table of the inspection system must be repeatable to a very high degree of accuracy. Repeatable tables are expensive and are difficult and costly to maintain. The problem has been exacerbated as surface mount devices are increasingly employed and board dimensions continue to shrink. As the spacing between interconnected wires becomes smaller, the demand for accuracy in the repeatable x-y table continues to increase, leading to ever greater cost.